Mixing devices are used for various technical applications. Optimization of mixing devices aims at reducing the energy used to obtain a specified degree of homogeneity. In continuous flow mixing the pressure drop over a mixing device is a measure for the energy involved. Further, the time and space used to obtain the specified degree of homogeneity can be important parameters when evaluating mixing devices or mixing elements. Static mixers are have been used for mixing of two continuous fluid streams.
High volume flows of gas are for example mixed at the outlet of turbofan engines, where the hot exhaust gases of the core engine mix with relatively cold and slower bypass air. In order to reduce the sound emissions caused by these different flows lobe mixers were suggested for example in U.S. Pat. No. 4,401,269.
One specific application for mixing of continuous flow streams is the mixing of a fuel with an oxidizing fluid, for example air, in a burner for premixed combustion in a subsequent combustion chamber. In modern gas turbines good mixing of fuel and combustion air can be a prerequisite for complete combustion with low emissions.
In order to achieve a high efficiency, a high turbine inlet temperature is used in standard gas turbines. As a result, there can arise high NOx emission levels and higher life cycle costs. These aspects can be mitigated with a sequential combustion cycle, wherein the compressor delivers nearly double the pressure ratio of a known one. The main flow passes the first combustion chamber (e.g. using a burner of the general type as disclosed in EP 1 257 809 or as in U.S. Pat. No. 4,932,861, also called EV combustor, where the EV stands for EnVironmental), wherein a part of the fuel is combusted. After expanding at the high-pressure turbine stage, the remaining fuel is added and combusted (e.g. using a burner of the type as disclosed in U.S. Pat. No. 5,431,018 or U.S. Pat. No. 5,626,017 or in US 2002/0187448, also called SEV combustor, where the S stands for sequential). Both combustors contain premixing burners, as low NOx emissions can involve high mixing quality of the fuel and the oxidizer.
Since the second combustor is fed by the expanded exhaust gas of the first combustor, the operating conditions allow self ignition (spontaneous ignition) of the fuel air mixture without additional energy being supplied to the mixture. To prevent ignition of the fuel air mixture in the mixing region, the residence time therein should not exceed the auto ignition delay time. This criterion can ensure flame-free zones inside the burner. This criterion can pose challenges in obtaining appropriate distribution of the fuel across the burner exit area.
SEV-burners are currently only designed for operation on natural gas and oil. Therefore, the momentum flux of the fuel is adjusted relative to the momentum flux of the main flow so as to penetrate in to the vortices. This can be done using air from the last compressor stage (high-pressure carrier air). The high-pressure carrier air is bypassing the high-pressure turbine. The subsequent mixing of the fuel and the oxidizer at the exit of the mixing zone is just sufficient to allow low NOx emissions (mixing quality) and avoid flashback (residence time), which may be caused by auto ignition of the fuel air mixture in the mixing zone.